N-acyl-homoserine lactones, known as autoinducers (AIs), are widely conserved signal molecules present in quorum-sensing systems of many Gram-negative bacteria. It has been found that AIs are involved in the regulation of a range of biological functions, including bioluminescence in Vibrio species (Eberhard et al., 1981; Cao and Meighen, 1989), Ti plasmid conjugal transfer in Agrobacterium tumefaciens (Zhang et al., 1993), induction of virulence genes in Erwinia carotovora, Erw. chrysanthemi, Erw. stewartii, Pseudomonas aeruginose, P. solanacerum, and Xenorhabdus nematophilus (Jones et al., 1993; Passador et al., 1993; Pirhonen et al., 1993; Pearson et al., 1994; Beck von Bodman and Farrand, 1995; Flavier et al., 1998; Costa and Loper, 1997; Nasser et al., 1998;), regulation of antibiotic production in P. aureofaciens and Erw. carotovora (Costa and Loper, 1997; Pierson et al., 1994), regulation of swarming motility in Serratia liquifaciens (Eberl et al., 1996), and biofilm formation in P. fluorescens and P. aeruginosa (Allison et al., 1998; Davies et al., 1998). Many more bacterial species are known to produce AIs, but the relevant biological functions have not yet been established (Bassler et al., 1997; Dumenyo et al., 1998; Cha et al., 1998). Biofilm formation is of particular significance to bacterial pathogenicity, as it makes bacteria more resistant to antibiotics and host defense responses, and causes microbial contamination in medical devices and in drinking water pipelines.
Different bacterial species may produce different AIs. All AI derivatives share identical homoserine lactone moieties, but differ in the length and structure of their acyl groups. Although the target genes regulated by AIs are extremely varied, the basic mechanism of AIs biosynthesis and gene regulation seems to be conserved in different bacteria. The general feature of gene regulation by AIs is cell density dependence, also known as quorum sensing. At low cell densities the AIs are at low concentrations, and at high cell densities the AIs can accumulate to a concentration sufficient for activation of related regulatory genes (Fuqua and Winans, 1996). The biological functions regulated by AIs are of considerable scientific, economic, and medical importance. New approaches for up or down regulation of bacterial quorum sensing systems would be of significant value, not only in science, but also in practical applications.
It has been reported recently that a novel gene encoding autoinducer inactivation (aiiA) has been cloned from the Gram-positive bacterium Bacillus sp. strain 240B1 (Dong et al., 2000). Expression of the aiiA in transformed Erw. carotovora strain SCG1, a pathogen that causes soft rot disease in many plants, significantly reduces the release of AI, decreases extracellular pectrolytic enzyme activities, and attenuates pathogenicity on potato, eggplant, Chinese cabbage, carrot, celery, cauliflower, and tobacco. The results indicate the promising potential of using the AI-inactivation approach for prevention of diseases in which virulence is regulated by quorum sensing signals.